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Outline of landforms and geology of Japan
Geology
Fig.
Simplified geological map [
]
Photo:
Complex
structure of strata []
Photo:
Crystalline schist (Nagatoro) []
Fig.
Basement geology of Japan []
Fig. Cross section of southwestern Japan []
Rocks in the Japanese Islands are roughly classified into pre-Neogene rocks, plutonic rocks including granite, and Neogene and Quaternary rocks. Pre-Neogene rocks constitute the Japanese Islands as basement rocks, covered with Neogene and Quaternary deposits and volcanic products. Plutonic rocks including granite intruded these rocks. Strata subjected to intense force have complex structures such as folds and faults. Horizontal strata are only seen in younger deposits in Japan.
Basement -- accretionary complexes and metamorphic rocks
Basement of the Japanese island arcs consists mainly of accretionary complexes produced from the Permian to the Jurassic (after 250 million years ago) and regional metamorphic rocks.
Accretionary complexes include, in ascending order, basalt, pelagic chert consisting mainly of radiolarian shells, siliceous mudstone, and thick sandstone and mudstone. Chert of various ages and basalt are contained in muddy matrices as fragments of all sizes (mélange) (see “Accretionary prisms”). In addition, accretionary complexes may have blocks of basalt and reef limestone derived from seamounts, and ophiolite. The seamounts were basaltic volcanoes with reef limestone on top, formed on the ocean seafloor far from a subduction zone and moved to a trench. Ophiolite is regarded as fragments of an oceanic plate and the upper mantle, consisting of ultramafic rocks (serpentinite), mafic rocks (gabbro, dolerite, and basalt), and chert.
Metamorphic rocks widely distributed are metamorphosed accretionary prisms, mainly crystalline schist characterized by the parallel arrangement of the constituent minerals (schistosity). Gneiss also occurs in metamorphic rock zones. Metamorphic rock in island arcs is briefly described in the section, “Metamorphic rock.”
These basement rocks are classified into belts based on their formation ages and geological characteristics. The basement belts are zonally distributed in order of age, generally which are younger toward the Pacific Ocean. Jurassic accretionary complexes most widely occupy the Japanese Islands. Most of the belts are bordered by thrust faults; an older belt overlies the adjacent younger belt. These features indicate that the basement rocks were produced in subduction zones (see “Japan in a subduction zone” and “Formation history of the Japanese Islands”).
The characteristics of basement belts are different in southwestern Japan, northeastern Japan, and Hokkaido.
[Southwestern Japan (from Kanto to Kyushu)]
The geologic structure is typical of a subduction zone. Basement rock belts are distributed clearly parallel to the Nankai Trough in order of age. However, the Chichibu Belt, a Jurassic accretionary complex zone is situated in the Pacific side of the Median Tectonic Line, to the south of the Mino-Tamba Belt which is also the Jurassic accretionary complex zone. This reason is explained in “Formation history of the Japanese Islands, Page 2 (Cretaceous)”.
